Hydraulic valve lifters have been utilized for some time so as to vary timing and duration of valve opening so as to provide more optimum engine performance at various operating conditions (i.e. so-called "lost motion" systems). One such system employing hydraulic valve lifters is disclosed in U.S. Pat. No. 4,615,306 entitled "Engine Valve Timing Control System" of Russell J. Wakeman, issued Oct. 7, 1986 (the entire contents of this prior patent being expressly incorporated hereinto by reference and referred to hereinbelow as "the Wakeman '306 patent"). In the Wakeman '306 patent, valve timing and valve opening duration are controlled via pressure pulses developed within the engine oil supply as a result of lifter operation. The valve lifters themselves include a collapsible hydraulic link controlled by a solenoid. In a particular embodiment (see FIG. 6 of the Wakeman '306 patent), a pair of pistons defines therebetween a chamber which communicates with the solenoid. As the lower piston is being moved up the cam's profile, oil is pushed out of the lifter into bleed passageways until the lower piston's displacement is to be transferred hydraulically to the upper piston as dictated by an electronic control unit (ECU), at which time the solenoid is energized thereby forming a solid hydraulic link coupling the motion of the lower piston to the upper piston which, in turn, actuates valve opening. Since the effect of such a system is to eliminate the gentle closing ramp associated with the cam, high valve closing velocities and associated noise and valve durability problems (i.e., excessive valve wear) may result. Hence, it would be desirable to damp the valve closure during final travel to its seat.
Valve damping functions for hydraulic valve lifters have been proposed in U.S. Pat. No. 4,452,187 entitled "Hydraulic Valve Lift Device" of Toru Kosuda et al, issued June 5, 1984; U.S. Pat. No. 4,347,812 entitled "Hydraulic Valve Lift Device" of Toru Kosuda et al, issued Sept. 7, 1982; and U.S. Pat. No. 4,671,221 entitled "Valve Control Arrangement" of Bernhard Geringer et al, issued June 9, 1987.
In Kosuda et al '187 and '812 a so-called braking chamber is disclosed as being annular with respect to a plunger, the latter having a slit which allows oil to flow thereinto from an oil feed chamber during an upstroke of the plunger. Upon a downstroke of the plunger, the oil in the braking chamber will thus flow into the oil feed chamber through the slit and oil feed ports thereby reducing the volume of the braking chamber. As the plunger is further lowered during its downstroke so as to render the oil feed ports completely closed, the slit will commence to restrict the flow of oil from the braking chamber to the oil feed chamber (due to a variable open area of the slit being presented during its movement) and, as a result, the pressure in the braking chamber increases so as to act against the further lowering of the plunger thereby braking downward motion of the same.
Geringer et al '221 proposes to brake the motion of the valve on closing by providing a ramp-shaped annular chamber which cooperates with a ring-shaped projection of the housing block so that when the valve piston is in its downstroke, the ramp-shaped chamber is increasingly closed by means of a gap between the projection and the ramps defining the chamber. The chamber thus narrows with increasing overlapping of the ramps and the face of the ring-shaped projection.
While Kosuda et al '187 and '812 and Geringer et al '221 provide valve braking or damping functions, hydraulic valve lash adjustment independent of the hydraulic link established between the pair of working pistons is unavailable. Geringer et al '221, in any event, cannot provide for hydraulic valve lash adjustment since a rigid mechanical connection exists between one of the working pistons in the Geringer et al '221 system and its associated engine valve. Kosuda et al '187 and '812, on the other hand, while having hydraulic valve lash adjusting capabilities, accomplish such valve lash adjustment in dependance upon pressurized oil in the lifter's pressure chamber--that is, in dependance upon the hydraulic link established between the pair of working pistons. What has been needed therefore is an improved hydraulic valve lifter which not only damps valve motion during its final travel (and thereby alleviates some of the problems associated with "lost motion" valve lifting systems) but which also adjusts valve lash hydraulically independent of the hydraulic link established between working pistons of the valve lifter. It is towards attaining such improvements that the present invention is directed.
In accordance with the present invention, a hydraulic valve lifter assembly is disclosed and claimed whereby valve damping and/or valve lash adjusting functions may be provided. And, the valve lash adjusting functions are achieved hydraulically independent of the hydraulic link established between its pair of working pistons. Those functions are provided (at least in part) by a lash adjusting piston and associated lash adjustment chamber whereby fluid may flow into same from a pressure chamber defined between a cam follower piston and a valve damping piston via an aperture in the latter. Thus, hydraulic displacement of the lash adjustment piston and concomitant adjusting of the valve lash occurs.
The lash adjusting piston, in a particularly preferred embodiment of the invention, is slidably received within a cylindrical cavity of an axially elongate flange of the valve damping piston so as to define therebetween the lash adjusting chamber in which a compression spring is disposed, the spring biasing the valve damping and lash adjustment pistons in a direction tending to separate the same. Moreover, one-way valve structure (e.g. a spherical plug) normally closes the aperture defined in the valve damping piston. Since the cam follower piston preferably defines a cam follower surface having a greater surface area as compared to the upper surface of the lash adjusting piston (which is adapted to cooperate with motion-transferring structures to open/close the engine valve), the force transferred to the cam follower piston at positions on the cam other than the cam's base circle is believed to be translated into a lesser pressure within the pressure chamber as compared to the pressure within the lash adjustment chamber. Thus, a solid hydraulic link is established between the cam follower piston on the one hand and the valve damping/lash adjusting pistons on the other hand during upstrokes and downstrokes of the latter. However, with the cam follower surface in contact with the base circle (and hence substantially equivalent pressures in the pressure and lash adjustment chambers), the bias force of the spring will urge the lash adjustment piston into zero lash relationship with the motion-transferring structure (e.g. rocker arm, push rod, etc.) and thereby unseat the one-way valve structure to open the aperture and allow for an additional amount of oil to flow from the pressure chamber into the lash adjusting chamber. In this way, valve lash is hydraulically adjusted independent of the hydraulic link between the cam follower and valve damping/lash adjusting pistons.
Primary and secondary fluid passageways establish fluid communication between the pressure and damper chambers and are closed via respective primary and secondary passageway-closing structures. In order to prevent motion damping from occurring during lifter upstroke, fluid is initially allowed to flow from the pressure chamber to the damper chamber via the secondary passageway when the valve damping and cam follower pistons first being an upstroke from their rest positions. Later in the upstroke, fluid flows into the damper chamber via both primary and secondary passageways. During a downstroke of the follower and valve damping pistons, the secondary passageway is closed and thus fluid flows from the damper chamber to the pressure chamber only via the primary passageway. Such fluid flow continues until the valve damping piston reaches a predetermined position during its downstroke (established when the primary passageway-closing structure closes the primary fluid passageway). The fluid remaining in the damper chamber thus damps further movement of the valve damping piston from its predetermined position to its rest position.
The improvements and advantages of this invention briefly mentioned above will become more clear after careful consideration is given to the detailed description thereof which follows.